Hot-melt adhesives are thermoplastic compositions which are solid at room temperature. When heated to a liquid or molten form, the hot-melt adhesive can be applied to a substrate. If a second substrate is placed on the hot-melt adhesive before it cools back to a solid, an adhesive bond can be formed joining the two substrates.
Especially hot-melt compositions based on styrenic block copolymers, tackifying resins, and plasticizing oils, have been employed in a wide variety of product assembly applications, mainly in the diaper or feminine napkins production for the manufacture of disposable soft goods. A particularly preferred application is their use in bonding lightweight materials such as, but not limited to, polyethylene or polypropylene substrates to paper, fabric, tissue, non-wovens, polyethylene or polypropylene substrates or to themselves. Specific applications for such prior art adhesives have included sanitary napkins, disposable diapers, surgical drapes, adult incontinent products, hospital pads and other products like these.
Multi-purpose adhesive compositions are those adhesives which can be used for more than one application. It should be understood that, for example, in the manufacture of most disposable diapers, today, several different adhesive applications are present. These adhesive applications include:
1) the use of adhesives in construction, that is, bonding the polyethylene to the nonwoven and absorbent pad; PA1 2) the use of adhesives for landing strips, that is, bonding a reinforcing layer of a polyolefin film to the polyethylene in the area opposite the tape tabs; PA1 3) the use of adhesives for elastic attachment, that is, bonding the elastic material to the polyethylene in either the leg and/or waist area; and PA1 4) the use of core adhesives, that is, applying an adhesive to the absorbent core to increase the strength of the core. PA1 5) for bonding a non-woven material to the backsheet of a diaper and, PA1 6) as a multi-purpose adhesive. PA1 (a) about 50 to about 150 parts by weight of a styrenic block copolymer or mixtures and/or modified and/or hydrogenated derivatives thereof; PA1 (b) about 20 to about 450 parts by weight of a tackifying resin which, when incorporated into a reference composition consisting of 100 parts by weight of a styrene-isoprene-styrene copolymer having a styrene content of 20-30% by weight, 250 parts by weight of said tackifying resin, 50 parts by weight of a paraffinic/naphthenic (70/30) extender oil and 2 parts by weight of a stabilizer consisting essentially of pentaerythrityl-tetrakis[3-(3,5-di-tert.butyl-4-hydroxyphenyl)-propionate] , leads to the following properties of said reference composition: PA1 a melt viscosity of about 60,000 mPa.s or less at a temperature of 120.degree. C.; PA1 a tan .delta. value of about 3.5 or less, wherein tan .delta. is defined as the ratio between the loss modulus and the storage modulus of said composition; PA1 an elastic retention on spandex fibers (300%) or natural latex rubber (200%) after 4 h at 40.degree. C. of about 70% or more; and PA1 a crossover temperature of about 95.degree. C. or less. PA1 (a) a Ring and Ball softening point (R&B) of about 50.degree. C. to about 150.degree. C.; PA1 (b) a mixed methylcyclohexane aniline cloudpoint (MMAP) of about 10.degree. C. to about 75.degree. C.; PA1 (c) a Di-Acetone Alcohol cloudpoint (DACP) of about 35.degree. C. or less; PA1 (d) a z-average molecular weight (Mz) of about 10,000 Dalton or less; and PA1 (e) a UV absorbance at 268 nm ranging from about 2.0 to about 5.0. PA1 an R&B softening point of about 50.degree. C. to about 150.degree. C.; PA1 an MMAP cloudpoint of about 20.degree. C. or less PA1 a DACP cloudpoint of about 0.degree. C. or less, and PA1 an Mz value of about 15,000 Dalton or less. PA1 (a) a styrenic block copolymer component, PA1 (b) a tackifying resin, PA1 (c) optionally, a plasticizing oil, PA1 (d) optionally, a stabilizer and PA1 (e) optionally, a viscosity modifying resin. PA1 (a): about 50 to about 150, preferably about 70 to 125, most preferably approximately 100; PA1 (b): about 20 to about 450, preferably about 150 to 300, most preferably approximately 250; PA1 (c): about 150 or less, e.g. about 5 to about 150, preferably about 30 to about 100, most preferably approximately 50; PA1 (d): about 4 or less, e.g. about 0.1 to about 4, preferably about 0.5 to 3, most preferably approximately 2; PA1 (e): about 100 or less, preferably about 50 or less, most preferably approximately 25. PA1 (a') An unvulcanized elastomeric block copolymer wherein the respective monomeric moieties are arranged in an alternating sequence having the general configuration A-B-A-B-A-B- or A-B-A. PA1 (a") A teleblock copolymer comprising molecules having at least three branches radially branching out from a central hub, each of said branches having polystyrene terminal blocks and an isoprene segment in the center. PA1 it has a tan .delta. value of about 3.5 or less, wherein tan .delta. is defined as the ratio between the loss modulus and the storage modulus of said composition; PA1 it shows an elastic retention on spandex fibers (300%) or natural latex rubber (200%) after 4 h at 40.degree. C. of about 70% or more; and PA1 it has a crossover temperature of about 95.degree. C. or less; PA1 it exhibits a melt viscosity at 120.degree. C. of about 60,000 mPa.s or less. PA1 (a) natural and modified rosins, such as, for example, gum rosin, wood rosin, tall-oil rosin, distilled rosin, hydrogenated rosin, dehydrogenated rosin, dimerized rosin, polymerized rosin and rosin derivatives of esters of polyhydric alcohols, including polar or otherwise modified rosins; PA1 (b) polyterpene resins generally resulting from the polymerization of terpene hydrocarbons, such as, for example, the mono-terpene known as pinene, in the presence of Friedel-Crafts catalysts at moderately low temperatures; PA1 (c) copolymers and terpolymers of natural terpenes, e.g. styrene/terpene, (alpha) -methyl styrene/terpene and vinyl toluene/terpene; PA1 (d) polar or otherwise modified terpene resins, e.g. phenolic-modified terpene resins such as, for example, the resin product resulting from the condensation, in an acidic medium, of a terpene and a phenol; PA1 (e) aliphatic petroleum hydrocarbon resins, the latter resins resulting from the polymerization of monomers consisting primarily of olefins and diolefins; PA1 (f) aromatic petroleum hydrocarbons; PA1 (g) aliphatic/aromatic petroleum derived hydrocarbons; PA1 (h) polar or otherwise modified aliphatic and/or aromatic petroleum hydrocarbon resins, such as, for example, copolymers and terpolymers of petroleum hydrocarbon resins with vinyl monomers, or mixtures thereof, such as vinyl arenes, vinyl pyridines, vinyl halides and vinyl carboxylates, including particularly those monovinyl aromatic hydrocarbons of the benzene series, such as, styrene, vinyl toluene, (alpha)-methyl-styrene, vinyl xylene, ethyl vinyl benzene, as well as dicyclic monovinyl compounds, such as, vinyl naphtalene and the like, as well as acrylic monomers, such as, acrylonitrile, methacrylonitrile, esters of acrylic acids, etc. Other modifying ingredients may be derived from alpha olefins, alkylene oxides, acetals, urethanes, natural rosins, phenols, etc. PA1 (a) A Ring and Ball softening point of from about 50.degree. C. to about 150.degree. C., more preferably of from about 60.degree. C. to about 140.degree. C, most preferably of from about 75.degree. C. to about 135.degree. C.; PA1 (b) An MMAP cloudpoint of from about 10.degree. C. to about 75.degree. C., more preferably of from about 20.degree. C. to about 70.degree. C.; PA1 (c) A DACP cloudpoint of maximum 35.degree. C., more preferably of maximum 25.degree. C., most preferably of maximum 15.degree. C; PA1 (d) An Mz value of about 10,000 Dalton or less, more preferably of maximum 5,000 Dalton, most preferably of maximum 3,000 Dalton; and PA1 (e) A UV absorbance at 268 nm (UV alpha) ranging from 2.0 to 5.0, preferably from 2.7 to 3.1. PA1 (1) Low molecular weight, that is, 1000-6000, polyethylene having a hardness value, as determined by ASTM method D-1321, of from about 0.1 to 120 and ASTM softening points of from about 65.degree. C. to 125.degree. C.; PA1 (2) Petroleum waxes such as paraffin wax having a melting point of from about 55.degree. C. to 100.degree. C. and microcrystalline wax having a melting point of from about 55.degree. C. to 100.degree. C., the latter melting points being determined by ASTM method D127-60; PA1 (3) Atactic polypropylene having a Ring and Ball softening point of from about 120.degree. C. to 160.degree. C.; PA1 (4) Synthetic waxes made by polymerizing carbon monoxide and hydrogen such as Fischer-Tropsch wax. PA1 (1) synergists such as, for example, thiodipropionate esters and phosphites; and PA1 (2) chelating agents and metal deactivators as, for example, ethylenediaminetetraacetic acid, salts thereof, and disalicylpropylenediimine.
The methods of application of these prior art adhesives have included, but are not limited to, spray or wheel application systems or (multi-bead or slot) extrusion. One of the most critical parameters during the application of the hot-melt adhesive is the "open-time" of the adhesive. "Open-time" is the time during which the adhesive is applied to a first substrate and remains sufficiently molten to effect a bond between the first substrate and a second substrate.
Once the open-time has been exceeded, the second substrate cannot be readily bonded to the first substrate, unless the adhesive exhibits a high degree of tack or pressure sensitivity, which, however, manifests itself in poor cohesive strength. Since sufficient cohesive strength of the adhesive is one of the key properties, such adhesives cannot be formulated as multi-purpose adhesive compositions. The adhesive must possess sufficient cohesive strength to provide high bond strength values when subjected to stress, either at ambient or elevated temperatures, so that the bonded parts cannot be easily separated. For example, it is very important that a construction adhesive for disposable soft goods maintains its bond not only at room temperature but also at elevated temperatures, that is 38.degree. C. (100.degree. F.). This elevated temperature resistance is important because without this characteristic, delamination of the end product occurs if the adhesive bond comes into contact with the user's skin. Apart from a high level of cohesion, long open-times are generally preferred for multi-purpose adhesives, because the wettability of the system towards the substrate is thereby improved, that is, the molten adhesive is allowed a longer period to spread out onto the substrate, which will improve its adhesion to the substrate.
As an additional criteria, it is necessary that the adhesive, upon application, is not absorbed into the actual disposable construction, and that the adhesive bonds not only remain secure, but also flexible even after prolonged periods of storage. In addition to requiring heat and oxidation resistance on aging, they should also possess a sufficient bonding range and must be light or clear in color. Moreover, since the adhesive is applied at high temperatures, excellent odour characteristics are required and it is of vital importance that the final compound does not irritate or sensitize the human skin to any extent.
Recently, many disposable garment manufacturers have begun to use spray application techniques. Spray application generally permits contact and ready application to uneven and irregular surfaces. Because spray application also allows for coating of less than the entire surface, spray application can reduce the amount of material used. Spray application is particularly useful where air or moisture permeability is desired. In addition there may be less risk of thermal damage to substrates than with other application methods.
Changes in diaper manufacturing have brought about the development of new hot-melt adhesives. Diaper manufacturers now use thinner gauge polyethylene to manufacture diapers. Not only does thinner gauge polyethylene produce a more aesthetically pleasant diaper but it also represents significant cost savings. However, as the gauge of the polyethylene is decreased, the likelihood of a burn through of these thin layers increases. In the past, the lower limit on the operating temperature was defined by the temperature at which the hot-melt adhesive could be applied with sufficiently low viscosity for even and proper application. Such temperatures were generally in the range of 150.degree. C. to 180.degree. C. It would be beneficial if hot-melt adhesives could be applied at temperatures of from about 120.degree. C. to about 150.degree. C.
It is therefore one of the objects of the present invention to provide a hot-melt adhesive composition which can be sprayed at such low temperatures.
Styrenic block copolymers are widely used to make hot-melt adhesives for a variety of uses, including diaper assembly. These styrenic block copolymers include unvulcanized elastomeric block copolymers wherein the respective monomeric moieties are arranged in an alternating sequence having the general configuration A-B-A.
In this configuration, A is a non-elastomeric block derived from styrene, usually referred to as styrenic "end-block", and B is an elastomeric polymer block derived from, for example, isoprene and/or butadiene, usually indicated as, for example, isoprene or butadiene "mid-block". This type of block copolymer may also be described as having a branched polymerized mid-block, derived from, for example, isoprene or butadiene, with a polystyrene terminal block at the end of each branch.
At temperatures of about 175.degree. C. or higher, adhesive systems based on styrenic block copolymers usually act as a homogeneous melt. The system is in the so-called "disordered" state. With decreasing temperatures, however, the polymer end-blocks, although connected to the elastomeric mid-blocks, have a strong preference to segregate from the mid-block phase, because of their inherent thermodynamic incompatibility. The mid-blocks and end-blocks are only compatible with each other at very high temperatures, such as, for example 175.degree. C. Because of this, the polymer mid-blocks and end-blocks start to segregate in two discrete phases during the cooling cycle of the adhesive, thereby inducing, a gradual, but significant increase in viscosity, plateau modulus, and cohesion of the adhesive, which appears to be more significant compared with systems which remain homogeneous during the cooling cycle.
It should be understood, however, that the mid-blocks and end-blocks of styrenic block copolymers remain connected to each other over the whole temperature range.
The segregation of the end-blocks is therefore better described by the phenomenon that the glassy polystyrene end-blocks start to associate into discrete "hard" domain structures, generally referred to as end-block domains.
This agglomeration process occurs because of their inherent thermodynamic incompatibility with the mid-block phase. These end-block domain structures act like a physical crosslink, thereby reducing the mobility of the adhesive, that is, they increase the viscosity, plateau modulus, and cohesive strength of the system. This process is temperature-reversible, that is, the domain structures will gradually disappear again when the temperature is increased. The glassy styrenic domains will soften and flow under heat and shear, temporarily disrupting the physical crosslink and allowing thermoplastic compounding and fabrication; upon cooling, the domains reform and the elastomeric character is restored. The domain formation process is usually called the "order-disorder transition" of the styrenic block copolymer. It starts at a specific temperature, that is, the temperature at which the first signs of incompatibility between the polymer mid-blocks and end-blocks become manifest.
The order-disorder transition of the system is accomplished gradually during the cooling cycle of the system and ends when all styrenic end-blocks have agglomerated into domains, that is, are grouped in an ordered state throughout the adhesive. Adhesives with a higher percentage of styrene as a total of the entire compound, usually exhibit a higher order-disorder transition temperature.
In some cases, certain tackifying resins are able to delay the order-disorder transition process, especially those containing a certain amount of polar or aromatic components, such as, for example, rosin esters, aromatically modified aliphatic resins or partially hydrogenated aromatic resins.
Tackifying resins which delay the order-disorder transition process, appear to postpone the significant increase in cohesion and viscosity caused by the mid-block and end-block phase segregation. It is believed that a larger amount of the styrenic end-blocks remains mobile in the system, or is presumably still partially compatible with the mid-block phase. This is highly beneficial for increasing the open-time of adhesives based on such styrenic block copolymers. For systems containing resins with only a limited amount of polar or aromatic components, the order-disorder transition process is usually completed at temperatures below approximately 45.degree. C.-50.degree. C. In this case, it is believed that the styrene end-blocks, which are incompatible with the mid-block phase at these temperatures, have all agglomerated into segregated rigid domains, acting like physical crosslinks which connect the polymer mid-blocks, thereby enhancing the cohesive strength and increasing the plateau modulus of the system.
It should be recognized that a higher percentage of styrenic end-block domains in the system will enhance the cohesive strength of the adhesive, will raise its plateau modulus and will improve its elevated temperature resistance.
However, it became evident following experimentation that the incorporation of tackifying resins and/or plasticizing oils containing a significant amount of polar or aromatic components into styrenic block copolymer based hot-melt adhesives often resulted in a significantly reduced cohesive strength and elevated temperature resistance of the adhesive. This effect appeared for all types of styrenic block copolymers, such as, for example, S-B-S (styrene-butadiene-styrene); (S-B)n (styrene-butadiene-styrene); S-I-S (styrene-isoprene-styrene); S-V-S (styrene-vinylbutadiene-styrene); S-EB-S (styrene-ethylene/butylene-styrene); and S-EP-S (styrene-ethylene/propylene-styrene) copolymers.
Although systems formulated with tackifying resins containing a significant amount of polar or aromatic components were found to have adequate adhesion properties in some cases, they showed a disadvantage because of their poor elevated temperature creep resistance and relatively low level of cohesive strength. As a result of the foregoing it was believed that adhesive compositions based upon styrenic block copolymers in combination with such tackifying resins would not be well suitable for use as multi-purpose adhesives.
It is therefore another object of the of the present invention to identify a resin useful as a tackifying resin in styrenic block copolymer based hot-melt adhesive compositions, which resin maintains or improves the adhesion, wettability and open-time of the system without significantly reducing the cohesive strength and elevated temperature resistance of the adhesive.
Hot-melt adhesives based on styrenic block copolymers such as, for example, "Kraton.RTM." thermoplastic rubber are said to provide excellent adhesion to non-polar substrates, such as polyethylene, which are commonly used in diaper manufacture. Additionally, these adhesives are said to maintain adhesion in elasticity demanding applications such as disposable diapers. Relatively low styrene content (from 15% to 24%) styrene-isoprene-styrene (S-I-S) block copolymers are said to be useful in adhesives for such applications.
In addition to the adhesives discussed above, adhesives based on styrene-butadiene-styrene block copolymers, such as S-B-S or (S-B)n copolymers, have been suggested for use in the construction of disposable soft. goods. Relatively high styrene content (from 25% to 50%) styrene-butadiene-styrene block copolymers are said to be useful in adhesives for such applications. Styrene-butadiene-styrene block copolymers, in particular S-B-S or (S-B)n block copolymers, or mixtures thereof, are hereafter abbreviated with the general designation S-B-S block copolymers.
The use of S-B-S block copolymers has proved particularly troublesome to the adhesive industry in terms of identifying a suitable tackifying resin. It has been known in preparing adhesives using S-I-S block copolymers that aliphatic tackifying resins may be used with success. Unfortunately, the aliphatic resins, while meeting the requirements for lower color adhesives, do not have adequate compatibility with S-B-S copolymers to form an acceptable adhesive.
It has been found that the desired adhesive properties are not present with aliphatic tackifying resins in S-B-S systems because of the higher mid-block solubility parameter component of S-B-S copolymers versus S-I-S copolymers. While aliphatic tackifying resins, with relatively low solubility parameters, are compatible with the mid-block of S-I-S block copolymers, they exhibit limited compatibility with the mid-block of S-B-S copolymers having a higher solubility parameter. The problem is even made worse by the use of oil in many formulations. If a more aromatic tackifying resin is used, a more compatible blend is formed with the S-B-S mid-block, particularly in the presence of oils. In order to maintain sufficient wettability to polyolefinic substrates adhesive compositions employing commercially available S-B-S copolymers were formulated with tackifying resins which exhibited a low to medium degree of functionality, such as partially hydrogenated aromatic resins, aromatically modified aliphatic C-5 resins or aromatically modified polyterpene resins, thus resins which showed adequate butadiene mid-block compatibility. These S-B-S based adhesive compositions appeared to be improvements over the previously employed adhesives in several important respects, but they also had several drawbacks which detracted from their usefulness.
For example, it was discovered that when an S-B-S based adhesive composition was left in an adhesive applicator for an extended period of time, it would rapidly increase in viscosity and ultimately gel thereby making its removal extremely difficult. Furthermore, adhesive compositions based upon S-B-S copolymers often showed poor adhesion properties, and did not appear to have sufficient elevated temperature creep resistance to perform well as an elastic attachment adhesive, or to bond a roamed elastic waistband to the disposable diaper, as compared with adhesive compositions based upon styrene- isoprene-styrene (S-I-S) block copolymers. Those skilled in the art will recognize that it is more difficult to tackify S-B-S copolymers than S-I-S copolymers. In this regard S-B-S systems are believed to possess a lower degree of adhesion, when compared with S-I-S adhesives.
In view of the prior art difficulties with formulating an appropriate multi-purpose S-B-S adhesive, one might think that adhesive compositions based upon S-B-S copolymers formulated with highly polar resins like, for example, rosin esters, could be suitable for that use. Rosin is a solid material that occurs naturally in the oleo resin of pine trees and typically is derived from the oleo resinous exudate of the living tree, from aged stumps and from tall oil produced as a by-product of kraft paper manufacture. After it is obtained rosin can be treated by hydrogenation, dehydrogenation, polymerization, esterification, and other post treatment processes. Rosin is typically classed as a gum rosin, a wood rosin, or as a tall oil rosin depending on its source. The material can be used unmodified, in the form of esters of polyhydric alcohols, or can be polymerized through the inherent unsaturation of the molecules. These highly polar materials are commercially available and can be blended into adhesives using standard blending techniques.
Representative examples of such prior art rosin derivatives, include pentaerythritol esters of tall oil rosin, gum rosin, wood rosin, or mixtures thereof. Adhesive compositions containing such highly polar rosin esters appeared to be improvements over the previously employed adhesives in several important respects, but they also had several drawbacks, which detracted from their usefulness. For example, it became evident following experimentation that resins which contain a significant amount of polar or aromatic components tended to show a weaker cohesive strength and less elevated temperature resistance. As a result, it was believed that adhesive compositions containing prior art rosin esters would not be ideally suitable for use as multi-purpose adhesives.
It is therefore another object of the present invention to identify a suitable tackifying resin and provide a hot-melt adhesive composition containing same, wherein even in an S-B-S based system the adhesion, wettability and open time are maintained or improved without significantly reducing the cohesive strength.
As discussed earlier, long open-times are preferred when the molten adhesive is applied to the substrate. Especially for application techniques such as spray application of the adhesive, longer adhesive open-times are very important. When the molten adhesive is sprayed, it will be understood that immediately after the adhesive composition is extruded through a nozzle, it is picked up by an air stream which transports the adhesive composition to the desired substrate.
This method of applying an adhesive tends to reduce the temperature of the adhesive composition, when compared to other application systems, even if the air stream is heated. This fast cooling effect usually induces a substantial decrease in the wettability and open-time of the hot melt adhesive. As a result of this, this particular method of application is more critical with respect to the viscosities and open-times.
Additionally the adhesive composition is substantially elongated by this process, which further enhances the cooling effect because the adhesive composition increases in its overall surface area. Because of this cooling effect, the open-time of the adhesive is markedly decreased, whereby the wettability of the adhesive, and consequently its adhesion to the substrate, is significantly reduced.
It is therefore an object of the present invention to provide a hot-melt adhesive composition having a longer open-time than previously known adhesives based on predominantly aliphatic resins, particularly when applied by a spray application technique.
In an attempt to improve the cohesion at body temperature of S-I-S based compositions, end-block reinforcing resins, such as, for example Endex.RTM. 155 Hydrocarbon Resin, were blended with same. However, these resins appeared to decrease the specific adhesion of the adhesive compositions to polyolefin substrates and also raised the raw material cost of the final adhesive composition inasmuch as these reinforcing resins are generally quite expensive.
Prior art adhesives containing end-block reinforcing resins also have had the noteworthy deficiency of an undesirable high viscosity and a reduced wettability and open-time of the system, making these systems unsuitable for spraying applications. This is shown below in Examples 22 through 25.
It is therefore another object of the present invention to provide tackifying resins and end-block compatible resins offering improved adhesion properties and open-times in styrenic block copolymer based hot-melt adhesive systems, and additionally providing enhanced cohesive strength at room and body temperature.
In addition to the adhesives discussed above, it has been suggested that adhesive compositions based upon high styrene S-I-S copolymers formulated with stryrenated polyterpene resins and/or highly polar resins like rosin esters, could be suitable for use as multi-purpose adhesives. Especially preferred were pentaerythritol esters of tall-oil rosin, wood rosin or gum rosin, or mixtures thereof. Relatively high styrene content (from 25% to 50%) styrene-isoprene-styrene block copolymers are said to be useful in adhesives for such applications. Such adhesive compositions appeared to be improvements over the previously employed adhesives in several important respects, but they also had several drawbacks, which detracted from their usefulness.
Namely, it became evident following experimentation that resins containing a significant amount of polar or aromatic components tended to decrease the elevated temperature resistance of the final compound significantly. In addition to this, these resins allowed to obtain only a reduced cohesive strength of the continuous phase of the adhesive wherein said continuous, phase comprises the polymer midblocks, the tackifying resin and the plasticizing oil. Thus, although these systems may possess adequate adhesion properties, they usually displayed a relatively poor elevated temperature resistance and a relatively low cohesive strength.
Furthermore, hot-melt adhesives based on highly polar resins, especially those based on the conventional rosin esters, tend to have poor odour characteristics in comparison with systems based on hydrogenated resins. Moreover, it has been speculated that some systems based. on conventional rosin esters may sensitize the human skin to some extent. It should be understood that systems based on rosin esters exhibit long open-times and excellent adhesion to polyolefinic substrates, but that the above mentioned drawbacks, which detracted from their usefulness, have urged the adhesive industry to seek for resins which can replace the conventional rosin esters in their adhesive formulations.
The present invention replaces the conventionally used rosin esters and styrenated polyterpene resins in styrenic block copolymer adhesive compositions, such as, for example, S-I-S formulations, by tackifying resins, which offer comparable adhesion properties and open-timers in S-I-S systems, and additionally provide enhanced cohesive strength at room and body temperature. In addition, these hot-melt adhesive compositions should exhibit excellent odour characteristics and should not irritate or sensitize the human skin to any extent.
Also, the present invention provides improved hot-melt adhesive compositions which are uniquely suited for the manufacture of disposable soft goods, particularly as a multi-purpose hot-melt adhesives for disposable diapers, feminine napkins and the like. They should further be particularly suitable for a spray application technique, even under lower temperatures.
Additionally, the present invention provides hot-melt construction adhesives, based on styrenic block copolymers or mixtures thereof, which have a high degree of specific adhesion without simultaneously suffering from a loss of cohesive strength, either at ambient temperatures or elevated temperatures. Additionally, the adhesives should possess a sufficient elevated temperature resistance, good wettability, long open-times and no tendency to increase in viscosity or gel even under prolonged heat aging at application temperatures.
Additionally, present invention provides hot-melt adhesives which are formulated in either single or multi-purpose adhesive products without the need of employing the reinforcing resins which were previously considered essential. The adhesives should form strong bonds to polyolefin substrates, elastic materials, that is, natural rubber latex, Lycra.RTM. or polyethylene elastic, and tissue and nonwoven substrates. Furthermore, they should have an improved creep resistance as compared to the prior art styrenic block copolymer based adhesives employed heretofore.
Hot-melt adhesives of the present invention are of utility for the purposes described above and which are durable, easy to apply utilizing conventional manufacturing techniques, and which further do not have the numerous shortcomings attributable to the prior art construction adhesives used heretofore.
Tackifying resins of the present invention impart a high degree of specific adhesion to styrenic block copolymer based hot-melt adhesives, and additionally provide longer open-times and enhanced cohesive strength both at ambient temperatures and elevated temperatures. The tackifying resin should impart these positive properties to any styrenic block copolymer based hot-melt adhesive composition, particularly to sytems based on S-I-S, S-B-S, S-V-S and (S-B)n block copolymers.